Understanding the nature, structure, and course of adolescent alcohol problems is necessary for meaningful research progress on the etiology and treatment of these problems. However, current nosological systems for alcohol use disorders have largely developed from research and clinical experience with adults. The primary goal of this research is to contrast the validity of different taxonomic systems for alcohol use disorders when applied to adolescents. This project will utilize all subjects who participate in PAARC'S Clinical Core assessment. The sample will contain a large number of male and female adolescents. Half of each gender will have a DSM-IV diagnosis of Alcohol Abuse or Dependence. Subjects without an alcohol diagnosis will provide a broad range of drinking and alcohol-related problems. Symptoms, diagnoses, independent problem measures, and alcohol and drug use will be assessed at baseline and several longitudinal follow ups. These design features provide standardized assessment of problems across a range of severity, with a developmental emphasis on staging and course. Measures include the Structured Clinical Interview for the DSM (SCID), independent measures of alcohol dependence and psychosocial dysfunction, and alcohol and drug use data. The SCID has been adapted to assess DSM-IV and ICD-10 symptoms, and other problem domains relevant to adolescents, providing a comprehensive assessment of alcohol symptomatology in this population. The reliability, covariation, diagnostic efficiency, and severity weighting of symptoms will be determined. We will examine which diagnostic criteria are most appropriate for adolescents. The staging and course over time of alcohol symptoms and disorders will be determined. Preliminary data suggest a new formulation of adolescent alcohol abuse, defined by heavy and dyscontrolled drinking with associated social and interpersonal problems. The concurrent and predictive validity of this definition will be contrasted with DSM-IV Alcohol Abuse and ICD-10 Harmful Use diagnoses. Other analyses will contrast the concurrent and predictive validity of different definitions of alcohol dependence. This research will yield important information on the development and nature of adolescent alcohol problems.